1. Field of the Invention
This invention relates to document storage and retrieval systems. In a primary application the invention relates to storing information by using a modulated light source to redistribute the material of a deformable film coating.
2. Description of Prior Art
A variety of file systems have been considered for storing many documents onto a high resolution material. Microfilm systems optically reduce the original and photograph it on a high resolution emulsion. Photographic systems of this type, including scanned photographic systems, require time-consuming wet photography. Thus new entries to the file take considerable time to be made available. In addition, portions of a photographic film storage medium cannot be separately addressed at different times because the development process normally encompasses the entire film. Thus new documents cannot be added to an existing photographic storage medium. Photographic storage appears best-suited to applications where the entire plate is exposed in a single interval such as U.S. Pat. No. 3,198,880 "Photographic Disc Reproduction System for Television Signals" issued to P. M. G. Toulon on Aug. 3, 1965.
One solution to the problem of having the stored information immediately available is described in U.S. Pat. No. 3,314,073 "Laser Recorder with Vaporizable Film" issued to Carl H. Becker on Oct. 20, 1964. In this system a thin thermally vaporizable film is coated onto a plastic substrate. An intense laser, focused on the coating, is used to vaporize the coating and produce arrays of transparent holes representing the information on being stored. These can be immediately read out using light sources imaged at the holes.
This system, however, has a number of practical problems. Firstly, the large energies required to vaporize the film can only be realized by very expensive, high power lasers at the desired writing speeds. Secondly the strip format of the recording material, in the form of motion picture film, results in relatively slow accessing times to specific regions. Also the lack of an accurate tracking method results in relatively low resolution and thus inefficient use of the area in recording data.
Rapid accessing times can be achieved using the disc format where the radial motion is used to find the desired document and the circumferential motion is used to rapidly read it out. Disc recording is normally used in applications where the entire disc is written on in a single sequence. For example in the recording of television programs, as is done in the MCA Disco-Vision system, the entire disc is written at one time and copies are made from it. In document storage systems, however, where individual documents are added at different times, the disc presents serious tracking problems. If the disc is removed and then remounted, the resultant eccentricities of the tracks from a slightly off-center mounting can cause the tracks to cross and thus be destroyed. If large guard spaces are used to avoid track crossing, the storage area is used inefficiently and a limited number of documents can be stored. In addition to eccentricities, various warping and distortions of the recording media can also cause the subsequently recorded tracks to cross with previously recorded tracks and thus destroy the stored information.
In the MCA Disco-Vision system, as with other disc recording systems, the information is recorded and played back at constant angular velocity. This makes inefficient use of the area of the disc and results in recording power problems. Much greater recording power is needed in the outer diameter parts of the disc because of the increased relative motion between the disc and the modulated recording light source. In addition, the inner diameters have resolution problems because the information is packed more closely together. A given data rate, at the reduced relative motion, results in greater packing density. A relatively large inner region of the disc is often unused because of this consideration.